1. Field of the Invention
The invention relates to a coating agent for plastic films, to plastic films coated with this agent and to the use of the plastic films so coated, in particular in transfer metallization.
2. Description of Related Art
In transfer metallization, thin metal coatings are transferred from a plastic carrier film to a substrate, for example a paper, to be metallized. The procedure followed is usually that a plastic film, for example a polypropylene film or a polyester film, is first coated with a thin metal coating. The metal coating is usually applied by a vacuum metallization technique. After metallization of the plastic carrier film, the latter is brought into contact with the substrate to be metallized, care being taken that the adhesion between the substrate to be metallized and the metal coating is greater than that between the metal coating and the plastic carrier film. This is usually effected by means of an adhesive, which is applied either to the substrate to be metallized or to the metal coating. After bringing substrate and metallized carrier film into contact and, where appropriate, after curing the adhesive, substrate and carrier film are separated again, the metal coating remaining on the substrate. The plastic carrier film can now be metallized again and used for a further transfer metallization process. Depending on the type and nature of the carrier film, this operation can be repeated several times.
Both the transfer metallization process itself and also suitable carrier films have been disclosed and are described in the art, for example, in EP-A-0,175,259, EP-A-0,208,257, EP-A-0,216,342, EP-A-0,266,650, DE-A-2,555,087, DE-A-2,856,510, DE-A-2,907,186, DE-A-2,747,241, and U.S. Pat. No. 4,153,495.
However, these known carrier films have numerous disadvantages. For example, polyester carrier films have such high adhesion to the metal coating, that the surface of this polyester film which is to be metallized must be provided with an adhesion reducing agent such as a release coating, in order to make transfer of the metal coating to another medium possible. In the case of multiple use of this carrier film, the surface to be metallized must be re-coated with the adhesion-reducing agent every time. For reasons associated with the process, this results in irregularities in the release coating in the edge regions of the carrier film and thus in uncontrollable variations in the metal adhesion. For this reason, a carrier film of this type has to be edge-trimmed after each metallization, which leads to ever narrower carrier films and thus ultimately restricts the number of times such a film can be reused. In addition, it is uneconomical for transfer metallization users if the width of the substrate to be metallized has to be adjusted every time to the varying width of the carrier film.
A further disadvantage is seen when the carrier film has double-sided metallization. This is usually the case because generally not only the metal coating to be transferred is applied to the carrier film but, in addition, a second permanent metal coating, which serves to dissipate electrostatic charges which result during the production and in particular during winding of the films, is applied to the rear of the carrier film. (The charge develops only if two different plastics are moved past one another.)
If there is no electrical discharge, so-called Lichtenberg figures appear on the metallized surface in the transfer step. Depending on the intensity of the electrostatic charge present, these may be more than just optical defects. In particular, the adhesion of the coatings to be transferred to the plastic web can also be increased to such an extent that delamination is subsequently no longer possible. This then leads to tearing of the carrier film and possibly damaging to the equipment used in the delamination process.
The use of polyolefin films also gives rise to a limitation in the processing temperature which can be used. As a result, long dwell times accompanied by long drying channels and transverse contraction of the film webs as a result of the tensile forces have to be taken into account if the temperatures during the process are too high for the films.
Furthermore, a carrier material is desired which has a higher resistance during curing of the two-component adhesive in the electron beam process than does the polypropylene most frequently used.